This invention relates to a moving picture compressing system and, more particularly, to efficient compressive-encoding which can cope with a variation of an attitude of a camera platform mounting a video camera.
Generally, a video signal has a large amount of data in comparison with an audio signal and is very often subjected to compressive-encoding when transmitted into a transmission line in order to prevent wasteful use of the resources, such as the frequency band.
For the purpose, various methods of compressive-encoding have been proposed. Among others, one standardized method is known as MPEG and another standardized method is defined in H.261 standardized by ITU-T for use in a videophone or a television conference.
In both the methods mentioned above, compressive-encoding has adopted motion compensation in order to increase a compression ratio and may be referred to as compressive-encoding with motion compensation.
In the compressive-encoding with motion compensation, a certain picture or frame which is selected as a particular picture is coded by referring to another picture as a reference picture.
Herein, description will be simply made about the compressive-encoding with motion compensation. At first, it is assumed that a common object is included in both the particular and reference pictures and is changed or moved in position in both the particular and the reference pictures. In this event, it is to be noted that both the particular and the reference pictures can be sufficiently decoded by the use of the reference picture and vectors which are representative of information concerned with positions between the particular and the reference pictures. Herein, such vectors are generally called motion vectors. From this fact, it is readily understood that motion compensation may be said as a technique which compensates motion of the object included in the particular picture by using motion vectors.
Herein, let consideration be made about extraction of such motion vectors from an image picked up by an image sensor or an image pickup device. In this case, such motion vectors are searched within a search area of the image sensor. Herein, it is noted that the search area has been fixed or has been determined by a complicated calculation. Such a complicated calculation may be exemplified by calculating a distribution probability of motion vectors within the search area. Specifically, judgement is made about whether or not the distribution probability of the motion vectors exceeds a predetermined value determined for an encoder. As long as the distribution probability exceeds the predetermined value, the search area is kept unchanged. Otherwise, the search area is not determined and, as a result, the motion vectors can also not be calculated in this image pickup device. Such techniques are disclosed in Japanese Unexamined Patent Publications (JP-A) Nos. 9-37269, 8-251592, 8-237660 and 7-193822, all which will be referred to as conventional techniques.
However, these conventional techniques are not always effective in all of uses and, as a result, effective compressive-encoding can not be often achieved in a specific use. Moreover, according to the conventional techniques, a compressive encoder is inevitably complex in structure to determine the search area.